gbproxy: Fix TC_dl_unitdata test failure introduced by 6ee0126971

This commit used a send template (ts) to match a received BSSGP PDU which
doesn't work due to some differences in the length field.
Use tr_BSSGP_IMSI again and change the template restrictions so it
compiles.

Fixes: 6ee0126971c83fd60658761263af559a482f13d9
Change-Id: I85676e96f8d32a9d2c7deadc1d66707b6b8697d0
diff --git a/library/Osmocom_Gb_Types.ttcn b/library/Osmocom_Gb_Types.ttcn
index 687a073..76988b5 100644
--- a/library/Osmocom_Gb_Types.ttcn
+++ b/library/Osmocom_Gb_Types.ttcn
@@ -1815,7 +1815,7 @@
 		}
 	}
 	template (present) PDU_BSSGP tr_BSSGP_DL_UD(template (present) GprsTlli tlli, template (present) octetstring pdu,
-					  template (omit) IMSI_BSSGP imsi := omit) := {
+					  template IMSI_BSSGP imsi := omit) := {
 		pDU_BSSGP_DL_UNITDATA := {
 			bssgpPduType := '00'O,
 			tLLI_current := tlli,